patch 9.0.1039: using a <Cmd> mapping CmdlineChanged may be triggered twice

Problem:    Using a <Cmd> mapping CmdlineChanged may be triggered twice.
Solution:   Count the number of times CmdlineChanged is triggered and avoid
            doing it twice. (closes #116820
diff --git a/src/ex_getln.c b/src/ex_getln.c
index 4bd200d..0eb7d86 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -1785,11 +1785,13 @@
 
 	if (c == K_COMMAND || c == K_SCRIPT_COMMAND)
 	{
-	    int	    clen = ccline.cmdlen;
+	    int	    cc_count = aucmd_cmdline_changed_count;
 
 	    if (do_cmdkey_command(c, DOCMD_NOWAIT) == OK)
 	    {
-		if (clen == ccline.cmdlen)
+		// Do not trigger CmdlineChanged below if the <Cmd> mapping
+		// already did that.
+		if (cc_count != aucmd_cmdline_changed_count)
 		    trigger_cmdlinechanged = FALSE;
 		goto cmdline_changed;
 	    }